Our braininess may have evolved thanks to less sticky neurons

2019-03-08 01:11:08

Zephyr/Science Photo Library By Clare Wilson OUR braininess may have evolved thanks to gene changes that made our brain cells less sticky. The cortex is the thin, highly folded outer layer of our brains and it is home to some of our most sophisticated mental abilities, such as planning, language and complex thoughts. Around three millimetres thick, this layer is folded into an intricate pattern of ridges and valleys, which allows the cortex to be large, but still fit into a relatively small space. Many larger mammals, such as primates, dolphins and horses, have various patterns of folds in their cortex, but folds are rarer in smaller animals like mice. So far, we have only identified a few genetic mutations that contributed to the evolution of the human brain, including ones that boosted the number of cells in the cortex. One theory about how the cortex came to be folded is that it buckled as the layer of cells expanded. Daniel del Toro at the Max Planck Institute of Neurobiology in Munich, Germany, and colleagues wondered if some of the genetic changes in our brain’s evolution might have been about more than just an increasing number of cells. They investigated the genes for two molecules – FLRT1 and FLRT3 – which make developing brain cells stick to each other more. “If brain cells are less sticky, they may form more complex structures within the cortex” Human brain cells produce only a small amount of these compounds, while mice brain cells make lots. Del Toro’s team created mice embryos that lacked functioning FLRT1 and FLRT3 genes, which meant their cortex cells were only loosely attached to each other, like those of humans. When the genetically altered mice were born and grew into adults, they had more folded brains. This finding was presented at the Cortex Evolution and Development conference in Copenhagen, Denmark, last month. But it is still unclear if these mice are smarter – that is what the team plans to study next. Why would less sticky brain cells lead to a folded cortex? As a human fetus develops, new brain cells are produced in the middle of the head, and then move outwards until they reach the surface and become the cortex. Del Toro speculates that the stickier cells in normal mice might make the cortex layer too rigid to deform into wrinkles. “Imagine a line of people running together holding hands,” he says. “At the end, they all arrive at the same time.” The looser bonding of human brain cells might also allow them to migrate at different speeds and so form more complex structures within the cortex, says del Toro. But it is too soon to conclude that this was a key change in our evolutionary history, says Todd Preuss at the Yerkes National Primate Research Center of Emory University, Georgia, because the study only compared humans and mice. “It would be interesting to see what these genes are doing in other primates and mammals.” This article appeared in print under the headline “How we got our folded brains?” More on these topics: